1. Technical Field
The present disclosure relates to the field of photolithography implemented for the manufacture of integrated circuits.
2. Description of the Related Art
Photolithography is currently used in microelectronics to transfer patterns present on a mask to a resin layer deposited on a substrate. To that end, a photolithography exposure machine is used, configured to project a photon beam focused on the resin layer through the mask. The resolution of the patterns formed in the resin is proportional to a dimension called “Critical Dimension” CD equal to k·λ/NA, where k is a coefficient lower than 1 linked to characteristics of the machine, λ is the wavelength of the photons of the beam and NA is the numerical aperture of the photon beam at the mask level. The critical dimension of a photolithography machine is the width of the smallest shape susceptible of being transferred to a resin layer by the machine.
Independently of this critical dimension, the photolithography machine settings are of great importance, and all the more since the critical dimension to be reached is low. Indeed, the depth of field of a photolithography machine is proportional to λ/NA2 (=k2·CD/NA, where k2 is a proportionality coefficient). Therefore, the lower the critical dimension, the lower the depth of field of the photolithography machine. The result is that the focus setting of the photolithography machine becomes more and more sensitive as the critical dimension is reduced. This focus setting affects the edge slope of the patterns formed in the resin layer. In addition, the intensity of the photon beam emitted by the photolithography machine, called “dose”, affects the accuracy with which the mask patterns are transferred to the resin layer, and in particular the width of these patterns. It is known to form test patterns on photolithography masks, to transfer these test patterns to a resin layer, and to measure the dimensions of the test patterns formed in the resin layer. The measures obtained allow focus and dose setting values of the photolithography machine to be obtained.
FIG. 1A schematically shows profiles of test patterns formed in a resin layer on an axis of increasing focus F values of a photolithography machine. At the center of the axis, a pattern profile having vertical edges obtained with a focus setting of the machine at an optimal value Fop is shown. At focus values lower than 50 nm and 100 nm at the value Fop (Fop−100 and Fop−50), the pattern profiles obtained have edges whose slope is higher than 90°. The base width of the pattern obtained is therefore lower than that of the upper face of the pattern. At focus values higher than 50 nm and 100 nm at the value Fop (Fop+50 and Fop+100), the pattern profiles obtained have edges whose slope is on the contrary lower than 90°. The base width of the pattern obtained is therefore higher than that of the upper face of the patterns.
FIG. 1B shows a variation curve C1 of the critical dimension CD of a test pattern at average height, as a function of the focus F setting of the photolithography machine. Curve C1 shows the shape of a parabola centered on the focus setting optimal value Fop. The curve C1 shows that a measured value of critical dimension CDm corresponds to two focus setting values Fop−Fm and Fop+Fm on each side of the optimal value Fop. These two values correspond to pattern profiles on the right and the left of the optimal pattern Fop on the axis of FIG. 1A. The result is that, in general, a single measure of critical dimension CDm limits the ability to determine a focus setting value to be made. There is indeed a doubt between two focus correction values +Fm and −Fm. Therefore, the higher the focus F setting value, the narrower the upper face of the pattern profile.
FIG. 2A schematically shows profiles of test patterns formed in a resin layer, on an axis of increasing dose D values emitted by a photolithography machine. At the center of the axis, a profile of pattern having a desired width at average height is shown. This profile is obtained with a dose setting of the machine at an optimal value Dop. At lower dose values of 0.5 and 1 mJ/cm2 at the optimal value Dop (Dop−0.5 and Dop−1), the width of the profiles at average height, of the patterns obtained is higher at the desired width. At higher dose values of 0.5 and 1 mJ/cm2 at the value Dop (Dop+0.5 and Dop+1), the profiles of the patterns obtained have a width at average height which is on the contrary lower than the desired width. Therefore, the higher the dose setting value, the narrower the width of the pattern profiles.
FIG. 2B shows a variation curve C2 of the minimum width of a pattern CD as a function of the dose D setting emitted by the photolithography machine. The curve C2 has the shape of a straight line having a negative slope. The curve C2 shows that a measure of critical dimension CDm corresponds to a single dose setting value Dm, and that the higher the dose emitted, the lower the critical dimension measured.
To avoid the doubt previously mentioned relating to the focus F setting, it has been suggested to measure the slope of the edge of a pattern. FIG. 3 shows a pattern formed in a resin layer and having a height H. To that end, it is known to measure the critical dimensions of a pattern BCD near the pattern base (for example at 10% of the pattern height H) and that TCD near the upper face of the pattern (for example at 90% of the height H). FIG. 3 also shows the critical dimension measure at average height MCD corresponding to 50% of the height H at which the measurements of FIGS. 1A, 1B, 2A, 2B have been taken. The measures of critical dimension TCD and BCD make it possible to determine the slope SWA of the pattern profile and the critical dimension MCD, and therefore to determine both the value and sign of the focus correction and dose correction to apply to the photolithography machine. Such measurements may be taken by scatterometry. However, these measurements require using complex libraries. They may also be taken by Scanning Electron Microscope SEM, but they are not accurate enough and reveal to be very long to obtain. In addition, such measurements cannot be taken on complex layers, i.e., corresponding for example to transistor gates or layers comprising Shallow Trench Isolation STI. When such measurements are taken on high layers, it may be desired to take in to account the thicknesses of lower layers which are not always homogeneous.